Nonwoven webs which are comprised of loosely assembled webs or masses of fibers bound together with an adhesive binder have many applications. These include paper towels, disposable diapers, filtration products, disposable wipes, and the like. Vinyl acetate based emulsions incorporating crosslinkable functionality are widely used in the preparation of these nonwoven products. One of the favorite crosslinking systems is based upon N-methylolacrylamide.
The emission of formaldehyde represents a worker safety and health issue, as well as a consumer issue, and there have been significant attempts to prepare adhesive binders having either reduced formaldehyde content or they are formaldehyde-free. Reduction of formaldehyde in vinyl acetate based emulsions has been achieved by using less favored reducing agents to the formaldehyde sulfoxylates, or by reducing the level of N-methylol acrylamide employed in the adhesive binder. Formaldehyde-free binders eliminate both the N-methylol acrylamide and the use of any formaldehyde emitting reducing agent. However, either method for producing vinyl acetate based emulsion polymers for nonwoven webs has been done so at the expense of performance and cost.
Representative patents disclosing adhesive binders having a post-curable comonomer such as N-methylolacrylamide are as follows:
U.S. Pat. No. 3,081,197 discloses a nonwoven binder incorporating an internal plasticizer and a post-curable comonomer such as N-methylolacrylamide.
U.S. Pat. No. 3,380,851 discloses a binder comprising an interpolymer of vinyl acetate/ethylene/N-methylolacrylamide for the preparation of nonwoven goods.
U.S. Pat. No. 4,449,978 discloses vinyl acetate/ethylene/N-methylolacrylamide/acrylamide systems for producing nonwoven webs. The advantage of using the N-methylolacrylamide/acrylamide blend has been the ability to reduce the amount of formaldehyde present in the emulsion and in the cured web without a substantial loss of performance.
U.S. Pat. No. 5,540,987 discloses vinyl acetate/ethylene/N-methylolacrylamide polymers for use in producing nonwoven webs having reduced formaldehyde content as well as reduced formaldehyde content in the cured web. Reduced formaldehyde content is achieved by using a redox system based upon a hydrophobic hydroperoxide and ascorbic acid. These systems result in formaldehyde contents significantly lower than those systems produced using a corresponding reducing agent incorporating no formaldehyde such as sodium metabisulfite or formaldehyde emitting reducing agents such as sodium formaldehyde sulfoxylate.
Several of the approaches to reduced formaldehyde content in nonwoven webs have been directed to actually prepare formaldehyde free nonwoven binders. The approaches pursued for the former include the use of crosslinkers based upon acrylamidobutyraldehyde dialkyl acetal, the methacrylamide derivative, the cyclized version, methyl acrylamidoglycolate methyl ether, allyl glycidyl ether, 3-chloro-2-hydroxypropyl (meth)acrylate and acrylamidoglycolic acid. The problems with these kinds of crosslinkers for nonwoven binders included poorer performance (lower tensile development), activation of the crosslinking chemistry at a pH which is corrosive to the commercial machinery, the requirement to use a more expensive and commercially unavailable novel self-crosslinking monomer and the instability of the self-crosslinking monomer.
The invention relates to improved vinyl acetate based emulsion polymers incorporating N-methylolacrylamide as a crosslinking component for construction of nonwoven webs. The emulsions polymers are prepared by emulsion polymerization of vinyl acetate, N-methylolacrylamide and optional monomers in the presence of a stabilizing system and a redox catalyst system comprised of an oxidizing agent and a reducing agent. The improvement for achieving reduced formaldehyde in the emulsion polymer resides in the use of a glycolic acid adduct of sodium sulfite (a proprietary reducing agent sold under the trademark Bruggolite FF-6) as the reducing agent. Lower formaldehyde levels are achieved in contrast to other formaldehyde free reducing agents such as sodium erythorbate and sodium ascorbate, and to sodium formaldehyde sulfoxylate, or any of the other reducing agents typically used in the preparation of a nonwoven binder employing N-methylolacrylamide as the self-crosslinking monomer.
There are significant advantages associated with the use of the glycolic acid adduct of sodium sulfite in producing vinyl acetate based emulsion binders containing N-methylolacrylamide. They include:
an ability to generate vinyl acetate-based emulsions which are highly suited for use in preparing nonwoven products having substantially reduced formaldehyde levels in the emulsion or latex; and,
an ability to generate nonwoven products having excellent resistance to solvents and water while at the same time having excellent adhesion to the fibers for providing enhanced tensile strength.